Determinations based on the measurement of the optical properties of a sample are in laboratories usually carried out on plates having a plurality of reaction wells. Photometric measuring is in such a case best performed through the plate in the vertical direction. The measuring light is directed to the plate either from above or from below (cf., for example, WO-82/00356).
Fluorometric measurement through a plate is not particularly recommendable, since in such a case there travels through the plate also a large amount of excitation light, which complicates the measuring of emitted light. In addition, background fluorescence possibly due to the material of the plate constitutes a problem. In most present-day fluorometers available for routine assays, excitation light is directed to the plate from above, and also emitted radiation is collected from above.
In luminometers, the light emitted from the sample is in general directed to the detector from above.
In certain fluorometric assays, the sample is inhomogeneous so that the concentration of the analyte being measured is highest at the bottom of the plate well. This is especially the case in certain samples containing cell cultures. In such a case it would usually be best to direct both the excitation light to the sample and the emitted light from the sample through the bottom of the plate, in order that the solution above the bottom layer should not disturb the measuring.
There are known fluorometers in which the measuring can be carried out either from above or from below (e.g. Series 7600 Microplate Fluorometer, Cambridge Technology, Inc., USA). In such an apparatus the source of light and the detector are fixedly mounted. Excitation light is directed from the source of light to the plate and emitted light is directed from the plate to the detector by means of optical-fiber bundles. The end of a fiber bundle may be placed either above or below the plate. One problem in these apparatuses is the high price of the fiber optics suitable for the purpose.